This multicenter study proposes to examine the efficacy of low dose inhalational nitric oxide (NO) in the treatment of premature infants with severe Respiratory Distress Syndrome (RDS). The primary purpose of this study is to determine whether doses of 1, 4, 10 and 20 ppm of inhaled NO will improve oxygenation in this group of newborn infants with severe respiratory failure. NO has been identified as an endogenous vasodilator which is responsible for the regulation of smooth muscle relaxation via stimulation of the formation of cycle guanosine monophosphate (cGMP). Early trials have been shown it to be useful in the treatment of the term infant with pulmonary hypertension by lowering pulmonary vascular resistance. In clinical studies of adults with Adult Respiratory Distress Syndrome (ADRS), NO has been found to enhance gas exchange by improving ventilation-perfusion matching in the pulmonary microcirculation and thereby decreasing intrapulmonary shunt. One unanswered question is whether there is a role for inhaled NO in the premature infant with respiratory failure due to RDS. Several recent studies have reported evidence of elevated pulmonary artery pressures in infants with severe RDS. In the atelectatic lung of the infant with severe RDS, diffusion of NO may occur preferentially across well-ventilated alveoli, increasing perfusion and improving the ventilation-perfusion ratio. Inhaled NO may improve oxygenation by either of these two physiologic mechanisms.